Sucker rod compensator for subsurface well pumps

ABSTRACT

A compensator located in a sucker rod string adjacent to a deep well pump and embodying upper and lower telescopically arranged devices, one of said devices being a piston mechanism, the other of the devices being a cylinder mechanism in which the piston mechanism is telescopically arranged. A liquid supply trapped in the cylinder mechanism and originating from the well bore fluid transmits linear motion between the piston and cylinder mechanisms, the liquid being forced in small increments past the piston mechanism out of the cylinder mechanism of each upward stroke of the sucker rod string and compensator, allowing the upper device to shift in step-by-step fashion progressively relatively upwardly of the lower device to increase the overall length of the compensator, until the travelling valve of the pump taps against the standing valve of the pump. When this occurs, the upper device overrides or moves slightly downward of the lower device, causing fluid in the well bore to flow into the cylinder mechanism through a check valve in the piston mechanism to incrementally replenish the liquid supply trapped in the cylinder mechanism.

United States Patent [72] Inventor Talmadge L. Crowe Houston, Tex. [2]] Appl, No 883,564

[22] Filed Dec. 9, 1969 [45] Patented June 22, 1971 [73] Assignee Baker 011 Tools, inc.

City of Commerce, Calif.

{54] SUCKER ROD COMPENSATOR FOR SUBSURFACE WELL PUMPS 20 Claims, 5 Drawing Figs.

[52] U.S.Cl 417/554, 417/260, 417/448 [51] int. Cl F04b 3/00,

F04b 47/08, F04h 21/00 [50] Field otSearch 417/554, 448, 432, 396, 260

[56] References Cited UNITED STATES PATENTS 2,383,934 9/1945 Crake 417/260 3,045,606 7/1962 Schmidt 417/448 Primary ExaminerLeonard H. Gerin Attorney Bernard Kriegel ABSTRACT: A compensator located in a sucker rod string adjacent to a deep well pump and embodying upper and lower telescopically arranged devices, one of said devices being a piston mechanism, the other of the devices being a cylinder,

mechanism in which the piston mechanism is telescopically arranged. A liquid supply trapped in the cylinder mechanism and originating from the well bore fluid transmits linear motion between the piston and cylinder mechanisms, the liquid being forced in small increments past the piston mechanism out of the cylinder mechanism of each upward stroke of the sucker rod string and compensator, allowing the upper device to shift in step-by-step fashion progressively relatively upwardly of the lower device to increase the overall length of the compensator, until the travelling valve of the pump taps against the standing valve of the pump. When this occurs, the upper device overrides or moves slightly downward of the lower device, causing fluid in the well bore to flow into the cylinder mechanism through a check valve in the piston mechanism to incrementally replenish the liquid supply trapped in the cylinder mechanism.

ge -N:

SUCKER ROD CQMPENSATOR FOR SUBSURFACE WELL PUMPS sucker rods, the efficiency of the pump is oftentimes comparal0 tively low because of the inability of the travelling valve of the pump to shift downwardly toward the standing valve of the pump to a sufflcient extent. Preferably, the travelling valve should move on its downstroke to an extent in which it is disposed closely adjacent to the standing valve of the pump. Such close spacing in not obtained in many cases for a variety of reasons, including improper initial spacing between the travelling and standing valves upon assembly of a pumping apparatus, or variations in the spacing between the travelling valve and standing valve caused by changes in fluid levels in the well, the temperature of the fluid in the well, and in the density of the fluid being pumped. The incomplete or insuffcient lowering of the travel valve on the downstroke results in reduction in efficiency, ranging from complete inability to pump the fluid caused by trapping of gas between the travelling and standing valves (a gas lock condition) to no loss of efficiency whatsoever, which would occur where the pump barrel fills completely with an incompressible fluid of consistent density, a condition occurring rarely in oil and gas well operations.

When pumping a compressible fluid, a portion of the downstroke is consumed in compressing a fluid below the travelling valve to a pressure somewhat greater than the hydrostatic pressure of the column of fluid in the tubing above the travelling valve. At this point, the travelling valve will open and fluid previously trapped or contained between the travelling valve and standing valve will be transferred relatively upwardly through the travelling valve for the remainder of the downstroke. It is evident that the amount of downstroke consumed before the travelling valve opens can vary considerably, since it depends on the hydrostatic pressure, the pump intake pressure, flow restriction, and the compressibility of the fluids entering the pump. It is only after the travelling valve opens that effective work can be performed. Accordingly, to obtain maximum possible pump volumetric efficiently, it is necessary that the unswept volume of the pump be held to a minimum; that is, at the bottom of the stroke, the travelling valve should be as close to contacting or bottoming out on the standing valve cage as is practical.

To avoid the above-noted difficulties, the sucker rod string can be adjusted in length to cause the travelling valve or pump plunger to actuallypound against the barrel at the bottom of each stroke. This procedure may produce a high volumetric efficiency, but it normally occasions premature fatigue failure of the pumping system in general and of the rod string in particular, producing high and somewhat prohibitive operating expenses.

The aforenoted difficulties are overcome in a sucker rod compensator as illustrated in U.S. Pat. No. 3,376,826. In that compensator, the length of the sucker rod string is automatically changed to the extent required to insure positioning of the travelling valve of the pump plunger closely adjacent to the standing valve of the pump at the bottom of the travelling valve stroke. That particular compensator transmits the reciprocation of the sucker rod string to the pump through a trapped body of a clean fluid, such as a body of oil, which is maintained separate from the well bore fluid. The clean fluid is trapped, despite the fact that parts of the compensator must move linearly with respect to each other, by the use of elastomer seals. Such seals have a short life, and also allow gas in the formation fluid to enter the chamber containing the clean fluid and commingle with the trapped clean fluid, resulting in the trapped fluid becoming partially compressible, and thereby reducing the efficiency of the sucker rod compensator. In addition, the structure of the tool is such tat it is relatively long and comparatively costly to manufacture.

By virtue of the present invention, a sucker rod compensator is provided that uses the well bore fluid itself as a medium for transferring the linear motion of the sucker rod to the pump. Elastomer seals, slidably sealing against relatively movable parts, are eliminated completely, and any gases that might leak into the well bore fluid trapped in the compensator are automatically purged. The automatic increase in length of the compensator is accomplished by permitting a slight leakage of the trapped fluid therein past a control piston disposed in a cylinder portion of the compensator receiving the piston, and such fluid may also be permitted to leak slightly past a sleeve constituting a portion of a head end of the cylinder in which the piston portion of the mechanism is telescopically received. When the compensator has achieved its maximum overall length as a result of its automatic elongation, such length is retained by an automatic replenishing of the well bore fluid that has previously leaked from the cylinder mechanism, the well fluid being caused to automatically flow back thereinto.

By virtue of the present invention, an improved compensator is provided that overcomes the disadvantages of the prior device illustrated in U.S. Pat. No. 3,376,826, it has a significantly shorter overall length, a substantially lower manufacturing cost, and a longer useful life.

This invention possesses many other advantages, and has other purposes which may be made more clearly apparent from a consideration of a form in which it may be embodied. This form is shown in the drawings accompanying and forming part of the present specification. It will now be described in detail, for the purpose of illustrating the general principles of the invention; but is is to be understood that such detailed description is not to be taken in a limiting sense.

Referring to the drawings:

FIG. 1 is a diagrammatic view of a sucker rod operated pumping system for pumping fluid from the well bore;

FIGS. 2a and 2b together constitute a longitudinal section and side elevational view through a sucker rod compensator embodied in the system shown in FIG. 1, FIG. 2b being a lower continuation of FIG. 2a;

FIGS. 3a and 3b are a diagrammatic section through the rod compensator of FIGS. 2a and 2b in combination with a pump, FIG. 3b being a lower continuation of FIG. 3a.

As disclosed in the drawings, a string of tubing A is disposed in a well bore B and has a known type of deep well pump C therewithin that includes a barrel l0 suitably secured or anchored in the tubing string, and a plunger 11 reciprocable in the barrel through upward and downward movement of a sucker rod string 12 connected thereto. As is well known, the sucker rod string may be connected to a suitable mechanism for effecting its reciprocation, such as a walking beam mechanism D, hydraulic pumping mechanism, or the like.

The outer barrel 10 of the pump C has a standing valve (FIG. 3b), including a valve seat 13 through which well bore fluid can flow upwardly into the barrel, downward flow of the fluid from the barrel being-prevented by a standing valve element 14 adapted to move downwardly into engagement with the seat. The pump plunger 11 is movable upwardly and downwardly in the barrel, containing a travelling valve device in the form of a ball valve element 15 adapted to shift downwardly into engagement with its companion seat 16 surrounding the inlet 17 of the plunger. During the downstroke of the plunger 1], its travelling valve l5, 16 moves downwardly toward the standing valve l3, 14, the latter closing and the fluid thereabove passing through the seat 16 of the travelling valve, unseating its valve element 15 in an upward direction and then passing upwardly through the openings 20 in a plunger partition 21 to a position thereabove.

When the plunger 11 is moved upwardly, its valve element 15 engages its seat 16, causing the plunger to lift the entire fluid column in the tubing string A and discharge a portion of it at the top of the well bore. The fluid in the plunger itself can pass through its upper ports 22 into the barrel MD thereabove and through the upper ports 23 in the guide portion 24 of the barrel through which the plunger rod 25 is reciprocable. During the upward movement of the plunger Hi, its travelling valve 15, 16 moves upwardly away from the standing valve l3, 14, the valve element M of the latter moving off its seat 113 and allowing well bore fluid to flow upwardly through the seat 13 and through openings 18 in a barrel partition 19 into the barrel.

For maximum volumetric efficiency, it is desirable for the plunger ill to move on its downstroke to a position in which its travelling valve 15, 16 is closely adjacent to the standing valve l3 14 of the barrel. Accordingly, on the upstroke, the plunger 11 will partake of a maximum effective travel in the barrel MP and will lift the maximum quantity of fluid that may have passed through the travelling valve seat 16 into the plunger and outer barrel thereabove. If the fluid contains a substantial quality of gas, assurance is had that the gas between the travelling and standing valves will be compressed sufficiently on the downstroke of the plunger to overcome the hydrostatic head of fluid in the tubing string A, for the purpose of unseating the travelling valve element and causing the fluid above the standing valve l3, 114, including the gas therein, to travel upwardly into and through the plunger 1111. if the travelling valve 15, 16 does not move downwardly to its maximum extent, then gas locking might occur under some conditions in which the travelling valve does not open on its downstroke, the plunger 1 i on its upstroke merely allowing the trapped gas to expand, the standing valve element M also remaining in its closed condition, with no additional fluid from the well bore flowing past it and into the pump barrel lit).

To offer assurance that the plunger H and the travelling valve H5, 16 will shift downwardly to the desired maximum extent, an automatically adjustable compensator 27 is included in the sucker rod string R2, as between the plunger rod and a sucker rod section 28. As disclosed, the plunger rod 25 is 7 preferably secured to a suitable length of polished rod 29, which, for example, may be 30 feet long, and this polished rod is attached directly or through a short pony rod (not shown) to the threaded pin 30 of a bottom sub or head 50 constituting a portion of the compensator apparatus. The threaded pin 32 of a top sub 51 of the compensator apparatus is secured to the sucker rod section 28 of the string R2, the sucker rod string 12 extending to the top of the hole. The top sub 5 )1 is threadedly secured to a piston rod 52 depending therefrom and having a lower threaded pin 53 threadedly securing it to a piston 54), the rod and piston being relatively reciprocable in a cylinder 55 composed of several sections. Thus, the cylinder includes the bottom sub section 50, the upper end of which is threadedly secured to a lower sleeve section 56, which is, in turn, threadedly secured to an upper sleeve section 57 providing an annular cylinder head portion 58 including an inwardly directed flange or seat 59 against which is held a sleeve 60, which is retained in the upper sleeve section and against the flange by a sleeve retainer 611 threaded into the upper end of the upper sleeve section 57. The sleeve 60 makes a sliding, sealing fit with the piston rod 52, allowing the latter to shift longitudinally relative to the sleeve, and also permitting a very slight fluid leakage past the piston rod 52. Leakage of fluid past the exterior of the metal sleeve 60 is prevented by a suitable side seal ring 62 disposed in an internal circumferential groove 63 in the upper sleeve section 57 and sealingly engaging the periphery of the sleeve 60. In the event that rotary motion or torque is to be transmitted through the compensator to the pump, a clutch 64 is provided of the dog type. As disclosed, axial lugs or clutch teeth 65 are integral with and depend from the top sub 511, meshing with upwardly directed teeth or dogs 66 integral with and extending upwardly from the sleeve retainer 61;

The fluid in the well bore is permitted to enter the annular cylinder space 67 between the piston 54 and piston rod 52, on the one hand, and the cylinder 55 and its cylinder head 58, on the other hand, through side ports 6% in the bottom sub or head 50 extending from the exterior thereof to a central passage 69 in the latter, such passage communicating with the interior of the lower sleeve section 56 below the piston 54, and with a passageway 70 extending from the annular cylinder space 67 above the piston to the cylinder sleeve 56 below the piston 54. Such passageway includes a valve seat 71 held against a downwardly facing shoulder 72 in the piston by a seat retainer 73 threaded into the piston, this valve seat being engaged by a ball valve element 74 therewith, which bears against a spring seat 75 receiving the lower end of a helical compression spring 76, the upper end of which bears against a downwardly facing piston shoulder 77. When the valve ball 74 is elevated from its seat, fluid can flow upwardly through the seat retainer 73, valve seat 71, and around the ball valve element 7% into the passage through the piston thereabove, flowing through the spring 76 and into a lateral passage branch 78 that opens into the annular space 67 between the piston and piston rod 54, 52 and the cylinder 55. Fluid is prevented from passing downwardly through the passageway 70 by the downward engagement of the ball valve 741 with its companion valve seat 71.

During the upstroke of the sucker rod string 12, the piston rod 52 and piston 54 tend to move relatively upwardly in the cylinder 55. However, the annular cylinder space 67 is filled with a liquid, which initially may be oil or the well bore liquid, such liquid being trapped in the annular cylinder space between the piston 54 and the upper cylinder head portion 58 of the compensator, and transmitting the linear motion through the piston and piston rod mechanism 54, 52 to the cylinder 55. Fluid in substantial quantity is prevented from leaking from the annular cylinder space 67 on each upward stroke of the rods 12 by a plurality of piston rings 79 mounted in ring grooves 80 in the piston and sealingly engaging the inner wall of the lower sleeve section 56 of the cylinder. These piston rings may be of any suitable type. As an example, they each may be of the split type, being deflectable laterally in the grooves 80 to conform to any irregularities that may be present in the wall of the cylinder sleeve section. Although they seal effectively against the wall of the cylinder sleeve section, they still pennit a slight leakage or bypassing of fluid past the multiplicity of piston rings 79 from the cylinder space 67 downwardly past the piston 54 into the cylinder below the piston. The metal sleeve 60, which has working clearance only with the piston rod 52, may also permit a slight passage of fluid from the annular cylinder space 67 in an upward direction past the sleeve 60 during the upstroke of the sucker rods 12 and rod compensator 27 forming a part thereof. The extent of fluid displacement form the annular cylinder space 67, however, is relatively small during each upward stroke of the string of sucker rods, compensator and travelling portion 11 of the pump.

In the use of the apparatus illustrated, the pump assembly C is connected to the polished rod 29, the latter being connected to the lower sub 5% of the compensator apparatus 27, the upper sub 51 being connected to the sucker rods 12 thereabove. A sufficient length of polished rod 29 is provided to maintain tension on the rod compensator 27 and to overcome friction of the pump plunger 11 on its downstroke. This assembly is run in the well bore on the sucker rod string 12, the pump being forced into its seat in the tubing string A upon reaching bottom. After the pump has been seated, the rod string 12 is spaced out and the usual polish rod 91 at the top of the hole is appropriately hung on the power mechanism D at the top of the hole, so that when pumping is started, the pump will be tapping bottom very lightly; that is to say, the plunger 11 will be tapping the barrel 10 lightly, or a coupling 92 on the plunger rod 25 will be tapping the barrel guide 241 lightly. At this time, it may be assumed that the compensator is fully telescoped, such as disclosed in FIGS. 2a and 2b, the clutch elements 65, 66 being engaged and permitting torque to be transmitted through the compensator 27, if required.

The power mechanism D at the top of the hole is then caused to reciprocate, effecting reciprocation of the sucker rods 12, to shift the plunger 11 in the barrel 10. When the pumping operation starts, the tubing string A will elongate as a result of increased hydrostatic pressure therewithin, due to the rising column of liquid and also due to a decreased hydrostatic pressure or head in the well bore annulus 100 surrounding the tubing string, reducing the buoyant efi'ect of the liquid in the well casing on the tubing string. In addition, the initiation of the pumping operation will increase the average temperature of the tubing string A, causing it to elongate, and such increased temperature will also tend to cause the sucker rod string 12 to elongate. However, the rod string 12 may also shorten as a result of the buoyancy of the liquid within the tubing string A. The overall net result is that the initial relative spacing of the travelling valve 15, 16 and standing valve 13, 14 of the pump will start changing with the first stroke of the pump, and will continue changing throughout the life of the well, the changes also being due to changes of density of the fluid in the well bore passing through the pump. in a typical case, the travelling valve may be spaced above the standing valve at the bottom of each plunger stroke.

The rod compensator 27, within he limits of the relative movement that can take place between the cylinder and piston structure 55, 54, 52, automatically compensates for the changing well conditions. On the upstroke of the sucker rods 12, the upward force required to lift the fluid in the tubing string A is being transmitted from the piston 54 through the load transmitting liquid in the annular chamber or cylinder 67 to the upper cylinder head 58, the piston and piston rod structure and the cylinder moving essentially upwardly as a unit to elevate the travelling valve 15, 16 with respect to the standing valve 13, 14. The pressure developed in the trapped liquid within the annular cylinder space 67 will cause some of the liquid to bleed past the multiplicity of piston rings 79 and also through the small working clearance space between the metal sleeve 60 and piston rod 52 completely out of the compensator 27 and into the well bore. Only a very small amount of liquid will actually be displaced from the annular cylinder 67 since the clearance space between the piston rod 52 and metal sleeve 60 amounts to a relatively small choke orifice in area, which is also true of the amount of liquid that can leak past the multiplicity of piston rings and into the bottom sub or head 50, and then out through its ports 68 into the well bore. As a result, the upward shifting of the piston rod 52 and piston 54 relative to the cylinder 55 is very small during each upward stroke of the mechanism. As an example, only sufficient liquid or other fluid will be able to transfer through the sleeve 60 and past the piston 54 and piston rings 79 during each complete upward stroke of the piston rod and piston as to result in an elevation of the piston and piston rod within the cylinder by approximately 0.05 inches. In other words, the rod compensator 27 will extend or elongate a comparatively short distance on each upward stroke of the sucker rods 12, the rod compensator 27, and pump C connected thereto. On the downstroke of the sucker rods 12, rod compensator 27 and pump C, essentially no transfer of liquid will take place, the trapped liquid in the cylinder space retaining the cylinder 55 in its relative position along the piston rod 52 and piston 54.

The elongation of the rod compensator 27 will continue on each upstroke, fluid transfer from the cylinder space 67 taking place to a small degree, allowing the overall length of the rod compensator toincrease by small increments, which, in the example given above, may be 0.05 inches per upstroke. The progressive incremental elongation of the rod compensator will actually result in the plunger 11 and the travelling valve 15, 16 occupying a lower position in the barrel 10 on each subsequent downstroke of the pump mechanism, until the pump valve rod coupling 92 engages or taps the guide 24, or the plunger 11 taps a shoulder 95 in the pump barrel.

When the tapping commences at the bottom of each stroke of the pump mechanism, the piston rod 52 and piston 54 will shift downwardly relative to the cylinder 55 by a short distance, because of the fact that the string of sucker rods 12 thereabove can still move downwardly by a short distance,

which, in the example given above, will be 0.05 inches. This will result in the piston 54 moving downwardly within the cylinder 55 by an incremental distance (0.05 inches), forcing the fluid in the cylinder below the piston 54 upwardly through the return passage 70, after unseating the check valve 74, and flowing into the annular cylinder space 67 above the piston, thereby effecting a resupply of liquid to such cylinder space. As a result of the downward shifting of the piston 54 along and within the cylinder 55 by the short distance, the overall length of the rod compensator 27 is decreased, and this decrease is equivalent to the amount of its elongation on each upstroke.

On the next upstroke of the sucker rod string 12, including the compensator 27 and the travelling valve l5, 16 of the p'ump,the rod compensator will again elongate by virtue of the transfer of a small amount of liquid from the annular cylinder space 67 past the piston 54 and its piston rings 79 into the cylinder below the piston, and also upwardly through the clearance space between the sleeve 60 and piston rod 52 into the well bore thereabove. However, on the downstroke, a light tapping of the coupling 92 against the valve rod guide 24, or of the plunger 11 against the barrel 10, will again occur, the compensator 27 shortening again by the fact of the downward movement of the piston within the cylinder, causing unseating of the check valve 74 and flow of a slight amount of well bore liquid through the passageway 70 back into the annular cylinder space 67.

As a result, the rod compensator 27 elongates and contracts, depending on the variable conditions in the well bore, Any excess stroke that will bring the valve rod coupling 92 into contact with the valve rod guide 24 will arrest further downward movement of the cylinder structure 55, resulting in a slight downward movement of the piston structure 52, 54 and transfer of fluid from below the piston 54 upwardly through the check valve 74, 71 into the annular cylinder space 67, to automatically shorten the compensator 27 and the effective length of the entire sucker rod string 12. Assurance is had that the plunger 11 will be lowered within its companion barrel 10 to the maximum desired extent on each downstroke of the sucker rods 12, insuring a complete upward sweep of the pump barrel 10 by the plunger on its upstroke.

Not only is assurance had that a maximum upstroke of the plunger 11 and the barrel 10 is secured, but the fact that the plunger is shifted downwardly to its lowermost position on each downstroke insures against gas locking, since the bringing of the travelling valve 15, 16 closely adjacent to the standing valve 13, 14 will insure sufficient compression of the gas between the two valves as to overcome the hydrostatic head of the fluid in the tubing string A above the travelling valve, effecting opening of the latter and upward passage of gas into the plunger 11. Accordingly, upon the upstroke of the plunger 11 and the upward shifting of the travelling valve l5, 16 away from the standing valve 13, 14, the latter will be opened, and additional fluid, including gas, if present, will unseat the travelling valve element 14and pass into the plunger 11 and the barrel l0 thereabove.

The compensator 27 automatically adjusts the plunger 11, so that it will merely lightly tap the barrel 10. In the example given above, it is only necessary to effect a downward travel of the piston rod and piston within the cylinder structure of about 0.05 inches to relieve the engaging force of the plunger on the barrel, and such light tapping is insufficient to effect any fatiguing of the pumping system and of the rod string. This light tapping is to be distinguished from the pounding action that has occurred heretofore when sucker rod lengths were used as to cause striking of the plunger mechanism 11 against the barrel l0.

The compensator 27 avoids the necessity for trapping a body of clean fluid in the annular cylinder space 67, using the fluid in the well bore as makeup fluid when the compensator has automatically elongated to its maximum length. The avoidance of a trapped and selfcontained body of clean fluid permits the compensator to be made of a shorter overall length and manufactured at a lower cost than the prior device disclosed in the above patent. Moreover, there are no elastomer seals between the relatively movable parts of the compensator apparatus. Instead, metal-to-metal seals are employed, which have a long life. Such seals may permit gas to leak thereby and into the confined cylinder space or chamber 67, but such gas leakage does not adversely affect the operation of the apparatus since the gas is constantly being purged from the cylinder space 67 by being permitted to leak past the piston rings 79, and also through the slight orificelike clearance space between the piston rod 52 and metal sleeve 10.

lclaim: y

1. In a compensator for varying the effective length of a rod string in a well bore: an outer cylinder device; an inner piston device in said cylinder device and shiftable longitudinally with respect thereto; said cylinder device and piston device providing a chamber therebetween adapted to contain a liquid for transmitting longitudinal motion between said devices; a portion of one of said devices to one side of said chamber being communicable with the well bore; and means for allowing escape of a portion of the liquid from the chamber into the well bore to produce an increase in the overall length of said devices.

2. in a compensator as defined in claim ll; and means for returning liquid from the well bore to said chamber in response to longitudinal movement of the compensator in the well bore.

3. ln a compensator as defined in claim ll; and means for returning liquid from the well bore to said chamber in response to longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.

4. [n a compensator as defined in claim 1; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston device and cylinder device.

5. In a compensator as defined in claim ll; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston and cylinder sleeve.

6. In a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston rod and cylinder head.

7. in a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston and cylinder sleeve and a leakage path defined between the exterior of said piston rod and cylinder head.

8. [n a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston and cylinder sleeve; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.

9. [n a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a-piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston rod and cylinder head; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.

10. In a compensator as defined in claim 1; said cylinder device including a cylinderhead and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston and cylinder sleeve and a leakage path defined between the exterior of said piston rod and cylinder head; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.

11. In apparatus wherein a pump is disposed in a well bore, said pump including a standing valve mechanism and a travelling valve mechanism thereabove reciprocable by a string of sucker rods for elevating the well production to the top of the well bore, the combination therewith of a compensator in the sucker rod string comprising an upper device connected to an adjacent sucker rod member and a lower device connected to the travelling valve mechanism; one of said devices being an outer cylinder device; the other of said devices being a piston device in said cylinder device and shiftable longitudinally with respect to said cylinder device; said cylinder device and piston device providing a chamber therebetween adapted to contain a liquid for transmitting longitudinal motion between said piston device and cylinder device to effect reciprocation of said travelling valve mechanism toward and from said standing valve mechanism; a portion of one of said cylinder device and piston device to one side of said chamber being communicable with the well bore; and means for allowing escape of a portion of the liquid from the chamber into the well bore to effect relative shifting of said upper device upwardly along said lower device to increase the overall length of said upper and lower devices and bring the travelling valve mechanism to a bottom position closer to said standing valve mechanism.

12. In apparatus as defined in claim 11; and means for returning liquid from the well bore to said chamber in response to longitudinal movement of the compensator in the well bore.

13. in apparatus as defined in claim ill; and means for returning liquid from the well bore to said chamber in response to longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.

14. In apparatus as defined in claim 11; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston device and cylinder device.

15. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston and cylinder sleeve.

16. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston rod and cylinder head.

17. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said pistonand' cylinder sleeve and a leakage path defined between said piston rod and cylinder head.

18. In apparatus as defined in claim I]; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston and cylinder sleeve; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.

19. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston rod and cylinder head; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.

20. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston and cylinder sleeve and a leakage path defined between said piston rod and cylinder head; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore. 

1. In a compensator for varying the effective length of a rod string in a well bore: an outer cylinder device; an inner piston device in said cylinder device and shiftable longitudinally with respect thereto; said cylinder device and piston device providing a chamber therebetween adapted to contain a liquid for transmitting longitudinal motion between said devices; a portion of one of said devices to one side of said chamber being communicable with the well bore; and means for allowing escape of a portion of the liquid from the chamber into the well bore to produce an increase in the overall length of said devices.
 2. In a compensator as defined in claim 1; and means for returning liquid from the well bore to said chamber in response to longitudinal movement of the compensator in the well bore.
 3. In a compensator as defined in claim 1; and means for returning liquid from the well bore to said chamber in response to longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.
 4. In a compensator as defined in claim 1; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston device and cylinder device.
 5. In a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston and cylinder sleeve.
 6. In a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston rod and cylinder head.
 7. In a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston and cylinder sleeve and a leakage path defined between the exterior of said piston rod and cylinder head.
 8. In a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston and cylinder sleeve; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.
 9. In a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the eXterior of said piston rod and cylinder head; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.
 10. In a compensator as defined in claim 1; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between the exterior of said piston and cylinder sleeve and a leakage path defined between the exterior of said piston rod and cylinder head; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.
 11. In apparatus wherein a pump is disposed in a well bore, said pump including a standing valve mechanism and a travelling valve mechanism thereabove reciprocable by a string of sucker rods for elevating the well production to the top of the well bore, the combination therewith of a compensator in the sucker rod string comprising an upper device connected to an adjacent sucker rod member and a lower device connected to the travelling valve mechanism; one of said devices being an outer cylinder device; the other of said devices being a piston device in said cylinder device and shiftable longitudinally with respect to said cylinder device; said cylinder device and piston device providing a chamber therebetween adapted to contain a liquid for transmitting longitudinal motion between said piston device and cylinder device to effect reciprocation of said travelling valve mechanism toward and from said standing valve mechanism; a portion of one of said cylinder device and piston device to one side of said chamber being communicable with the well bore; and means for allowing escape of a portion of the liquid from the chamber into the well bore to effect relative shifting of said upper device upwardly along said lower device to increase the overall length of said upper and lower devices and bring the travelling valve mechanism to a bottom position closer to said standing valve mechanism.
 12. In apparatus as defined in claim 11; and means for returning liquid from the well bore to said chamber in response to longitudinal movement of the compensator in the well bore.
 13. In apparatus as defined in claim 11; and means for returning liquid from the well bore to said chamber in response to longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.
 14. In apparatus as defined in claim 11; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston device and cylinder device.
 15. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston and cylinder sleeve.
 16. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable along said sleeve and a piston rod connected to said piston and slidabLe in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston rod and cylinder head.
 17. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston and cylinder sleeve and a leakage path defined between said piston rod and cylinder head.
 18. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston and cylinder sleeve; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.
 19. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston rod and cylinder head; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore.
 20. In apparatus as defined in claim 11; said cylinder device including a cylinder head and a cylinder sleeve; said piston device including a piston slidable along said sleeve and a piston rod connected to said piston and slidable in and along said head; said means for allowing escape of a portion of the liquid from the chamber comprising a leakage path defined between said piston and cylinder sleeve and a leakage path defined between said piston rod and cylinder head; and means for returning liquid from the well bore to said chamber in response to the longitudinal movement of the compensator in the well bore comprising a check valve in said piston device permitting liquid to flow from the well bore to said chamber, but preventing flow of liquid from said chamber to the well bore. 